[Mosaic GPU] Load LLVM lowering interfaces for all dialects
Apparently we were missing interface registration code for LLVM lowering,
which the gpu-to-llvm pass gracefully ignores unless compiled with debug
assertions enabled. But, simply adding the assertions in fact makes the
pass _too powerful_ and makes it lower _all dialects to LLVM_, which is not
what we want. That's why I've replaced it with a minimal version that is
only repsponsible for handling the GPU dialect, making the lowering similar
to the one prior to extra registrations.
PiperOrigin-RevId: 641874183
